In electrical machines, permanent magnets (PM) are a well-known alternative to field windings for producing flux in an air gap between the armature and the field. PM machines are known for having a higher power density than other electric machines. However, field weakening or field adjustment is not easily accomplished in PM machines, because connections to field windings are unavailable in basic PM machines. As used herein, the generalized term "machines" shall mean and include both motors and generators. The term "PM electric machines" includes AC induction machines, DC machines and synchronous machines.
Field weakening is necessary for operation of the above-mentioned machines in the constant horsepower range, which commences at a speed known as "base speed." Below this speed, torque and speed are increased with armature voltage. At rated horsepower, armature voltage and speed cannot be increased without other adjustments. Instead, the field must be weakened to allow the above-mentioned machines to increase speed while sacrificing torque.
For PM generators, field weakening is used to control the generator output voltage at speeds above base speed. Otherwise, it is an option to use a buck-boost converter to control the DC link voltage of the inverter or converter. The technology for directly controlling the PM generator terminal voltage and the PM motor back electromotive force (emf) has not been available.
Various relatively complex approaches exist for obtaining field weakening of PM machines. One approach assumes that direct control of the magnetic flux is not available, and that field weakening should be accomplished by controlling the direct-axis armature current to weaken the air gap flux. In this approach, demagnetization of the permanent magnets can be a problem, due to the reaction of the armature to the direct axis current component.
Another approach has suggested an optimal high-saliency interior PM motor design for machines requiring a wide field weakening range.
Phase angle control has also been suggested for field weakening in PM machines. Although torque at no-load speed can be increased significantly by advancing the phase angle of operation, this results in increased power losses in the machine.
Another approach uses adaptive control of a surface mount PM motor over its entire speed range. In the adaptive flux-weakening method, the direct-axis current can be controlled for a variety of operating conditions.
Still another approach has suggested that with optimum alignment of the stator and permanent magnet fields, maximum torque per ampere is achieved up to base speed. Operation at higher speeds with reduced torque is achieved by adjustment of the current angle to reduce the effective magnetic flux, i.e., the equivalent of field weakening.